1. Field of the Invention
This invention relates to improvements in on-the-fly, free-flight hammer, impact-printing machanisms. The improvements are particularly useful in high-speed line printers employing a number of identical printing units of the impact type. More particularly, the invention relates to a lightweight, easily-manufactured hammer module-actuator module combination providing high print quality at low cost with minimal service requirements. The invention further pertains to a simple, low-cost, hammer-bank shifting mechanism to be used in conjunction with a set of lightweight hammer modules.
2. Description of the Prior Art
On-the-fly, high-speed impact printers designed for use as output devices in computing systems are well known in the prior art. They are usually operated by electrical signals originating from a computer or peripheral device to energize actuators which cause print hammers to strike a moving record medium. One class of high-speed impact printers are of the back-printing type wherein type characters are provided on a drum, disk or belt which is moved in front of the record medium on which printing is affected by striking from the back. The record medium is itself being continuously fed forward as each line is printed.
Since individual line printers consist of over 100 printing positions and sometimes as many actuators and hammer devices, cost-savings in any aspect of an individual actuator and/or hammer device rapidly multiply into a much larger, cost-saving per printing unit. Furthermore, to reduce costs, most line printers employ a shifting hammer bank so that any given hammer-actuator combination can print in several adjacent columns, thereby reducing the number of hammer-actuators needed. Thus, a heavy, cumbersome hammer-actuator made mostly of machined metal requires expensive, malfunction-prone linkage and typically a DC servo motor to perform the shifting.
Impact printers employing moving type require that the print hammer strike the moving record medium normally and retract immediately to avoid smears caused by the movement of the type and the record medium. Furthermore, high impact momentum is desired so as to produce clear multiple-copies.
The prior art has employed hammers which are theoretically cheaper and simpler to implement than slidable linear hammers. Their inability to provide normalcy and high impact momentum caused poor character coverage due to variations in forms thickness and/or use of multiple-copy forms. This produced, for example, bottom-heavy coverage for single part (or thin) forms and top-heavy coverage for six-part (or thick) forms. Furthermore, shifting of the hammer pivot by as much as 0.005 inch around its nominal centerline as a result of motion along the direction of movement of the type belt, once the hammer begins to contact the form, resulted in character coverage variations within a line of print. Finally, the tight tolerances required to control the hammer pivot location add extra cost and adjustments. Thus more recent prior art has employed linear motion slidable hammers to overcome these problems.
A typical prior art impact-printing, slidable-hammer, actuator mechanism, such as disclosed in U.S. Pat. No. 3,964,384, uses over 20 components per printing position, many of which are constructed of highly machined steel parts. This mechanism furthermore requires complicated assembly procedures using no fewer than five screws and a variety of pins in the fabrication of the actuator and hammer components. A second example is also entirely made out of metal parts which are subject to extensive machining. As disclosed in U.S. Pat. No. 3,726,213 it comprises some 30 separate pieces entailing considerable assembly cost per printing position. Even individual hammer assemblies known in the prior art such as that disclosed in U.S. Pat. No. 3,745,917 utilize over twelve machined pieces, including six fasteners per hammer. Each of the above two hammer, actuator mechanisms provide for extensive adjustment means thereby requiring continuous monitoring and maintenance throughout their useful lives. As a typical hammer bank must undergo 150 million cycles before refurbishing, such considerations are important.
The prior art recognized the fact that in a line printer employing a multitude of hammers, the repetition rate of a given printing position is determined by the cycle time of the actuator moving the hammer position and that a minimum time must elapse between the printing of two successive characters by a given actuator. Therefore the prior art typically has employed means to provide a given hammer with a set of multiple actuators and/or shifting means so that a given hammer and actuator can print in multiple columns. The former method, for example, used pivoted push rods; say where one hammer can be struck by three actuators. Since such a method requires the alignment of three assemblies, it results in costly structures and set-ups and entails many adjustments that periodically need re-setting.
When moving type is mounted directly on a high-mass carrier such as a print drum or a print disk, a high-mass hammer may be employed without causing vibrations of the type carrier and it is possible to effect relatively long contact times. When a low mass, flexible band or belt is employed as the type carrier, the band moves on an air film which requires the print hammer to force the type carrier through the air film before sufficient pressure is applied to the type to cause printing on a multiple copy record medium. An additional factor is that an increase in contact time increases the tendency to smear, which is normally compensated for by low band speeds.
Thus a compromise must be reached between the use of low-mass hammers thereby not perturbing the moving type and the use of high-mass hammers providing the desired impact momentum to produce clean multiple copies and good character coverage. Typically, low-mass high impact-speed hammers are chosen as providing the best performance especially if the hammers have a free-flight component to their travel.
The quality of impact printing suffers from the fact that different characters present different surface areas to be printed resulting in non-uniformity of darkness. The prior art has dealt with this problem, for example, by utilizing complex controls on the actuator-driven solenoid to deliver different impact energies for different characters.
On a related matter, businesses make use of multiple-copy forms which also present different print-energy requirements. When producing multiple-copy forms, prior art printers used forms compressors to eliminate the so-called "first character up" problems wherein the first few characters to be printed on a line are the lightest character on the line since they have to do the most work in compressing the forms. In other printers, hammer mass and/or velocity was increased to overcome this forms resistance. However such practice led to excessive embossing or cutting on single part forms.
Orginially line printers were designed to have one print hammer and one electronic driver for each printed column (generally spaced at ten columns per inch). Within the last decade, many printers have been built in which all or part of each actuator-driver is made to print in more than one column, as mentioned above, resulting in lower cost and lower output speed. Some of the techniques used are:
a. sharing each electronic actuator-device with two or more print hammers;
b. placing the print hammers at every other, or every third, etc. column and horizontally incrementing the record medium being printed on until all the columns are printed;
c. allowing each hammer face to span two or more columns (this technique requires the character font spacing to be equal to or greater than the hammer face width);
d. similar to technique "b", but incrementing the hammer bank instead of the medium.
The present invention relates to technique "d". In the prior art, various techniques are used to implement the scheme, such as incorporating a pushrod into the actuator assembly and allowing the pushrod to pivot. Such techniques generally employ complex mechanical devices involving substantial cost in both their materials and assembly. Furthermore these devices are prone to malfunction and generally require periodic monitoring and maintenance.
Another prior art technique was to utilize a relatively small number of print hammer actuators because of their cost, size and weight and through the use of expensive, complicated, fast-acting d.c. servo motors together with mechanical linkage perform a number of shifting increments within each printed line. The prior art put the designer to a clear tradeoff between complexity and cost, and speed.
A current limiting resistor in series with the solenoid in the actuator was commonly employed in the prior art; it allowed a higher voltage to be used to improve the drive circuit response. A significant amount of energy is dissipated in this resistor making the printer much less energy-efficient than it could otherwise be.